1. Field of the Invention
The present invention relates to a method and an apparatus for recording information including secret information, effective for recording a content necessitating protection of copyright and a method and an apparatus for reproduction thereof.
2. Description of the Related Art
Recently, any type of information can be digitized and technology capable of distributing such information through transmission media or recording media has been developed, as expressed by the term “digital revolution.” As a result, a great number of people have come to acquire digital information freely. In such an environment, signals such as digital audio signals, digital video signals, relating data which computers handle are recorded in a recording medium. Information transmission and storage are carried out so that the above-described signals are reproduced from the recording medium, information is copied to a read-only medium, transmitted information is reproduced or information is transmitted through a transmission line.
Recently, as a recording medium capable of recording a large volume of video/audio information, the digital versatile disc (DVD) has been realized. A movie over two hours long is recorded in a DVD and such DVD recorded information is reproduced through a playback apparatus, so that the movie can be watched freely at home.
DVDs are classified into: read-only DVD-ROMs, DVD-Rs which allow a one-time recording, and DVD-RW, DVD-RAM which allow re-recording.
DVD-video is a standard currently used which allows a whole movie to be recorded in a single disc. A user can acquire information based on digital signals freely through reproduction of such DVD-video discs or reception of digital broadcasting. Under such circumstances, if the acquired digital signals are copied to a recording medium such as a hard disc and the aforementioned DVD-RAM and encoded with an encoder based on the DVD-video standard, it is possible to copy a disc.
Thus, in a DVD-video, digital information to be recorded is encrypted. The copy protect method employing cryptography technology functions effectively for a DVD-video disc or DVD-ROM, in which encrypted information is pre-recorded.
In the field of such information transmission and storage processing, recently, copyright protection is gaining in importance. Particularly, if information necessitating protection of copyright is recorded on an ordinary recording medium, illegal copy needs to be prevented. That is, although an individual having a copyright permits recording of information to only a single recording medium, it is possible to illegally copy the information onto a number of recording medium, thus preventing this is of the utmost importance.
Because a rewritable or re-recordable recording medium has appeared, generation control information is needed for the content, and its control information includes “copy is prohibited,” “a single copy is permitted,” “copying several times is permitted” and the like.
Recently, as a concept for content control, such concepts as “move,” “check-out,” and “check-in” have appeared. These words are defined as follows.    (1) “Move”: Moving the content from a recording area to another recording area. Consequently, the content written into an original recording area is erased.    (2) “Check-out”: Copy is permitted N times or the content is copied from a recording area (or first recording medium) to another recording area (or second recording medium). The content written into an original recording area is not erased but copy control information N is reduced by one. Of course, if N=0 is reached, the content cannot be copied any more. Although usually, a copied content can be reproduced, this copied content is not permitted to be copied further.    (3) “Check-in”: When the copy control information of the content in an original recording area is for example, (N−1) times, executing processing for returning that content from the other recording area recorded previously. As a result, the copy control information for controlling the content of the original recording area is increased by one, i.e., changed to N times. It follows that the content written into the other recording area is erased.
If an information recording/reproducing apparatus, which operates according to one of the above concepts is used faithfully to its function, there occurs no problem. However, the information recording/reproducing portion may be modified so that the copy control information is made meaningless. Thus, the presence of the copy control information becomes meaningless.
Technologies for blocking such illegal copy of digital signals are disclosed in Japanese Patent KOKAI Publications No. 9-128990, No. 8-204584, and No. 8-28684.
The Japanese Patent KOKAI Publication No. 9-128990 discloses a method of recording by replacing a part of an error correction code with specific information (encryption key or the like). According to this method, since a part of the digital data or the error correction code is replaced with the specific information, an error occurs in the replaced portion. Therefore, if the amount of the specific information increases, the error rate of the original data increases, which increases the load on error correction processing.
According to Japanese Patent KOKAI Publication No. 8-204584, when supplying data subjected to error correction processing to a decoding portion, a correction impossible data portion is replaced with a special code containing a synchronous code so that it can be detected by the decoding portion. The decoding portion recognizes the error portion using the special code to carry out decoding.
According to the Japanese Patent KOKAI Publication No. 8-286840, encryption is carried out by changing additional information or the position of the additional information in order to prevent illegal copying. The allocation structure of digital data is determined so as not to occur a correction impossible state of digital data due to a data error generated by embedding encryption key information.
To control the content and control information (including copy control information), encryption and decryption processing is carried out for the content and control information in the information recording/reproducing portion. However, key information for encrypting or decrypting the content or control information may be illegally removed. Consequently, decrypting of the content and control information is enabled, so that it is possible that the content is duplicated illegally into another recording medium or recording area in a large quantity and the content is decrypted.
In order to prevent such illegal copying, procedures for encryption and decryption need to be improved further. Additionally, the recording medium for recording the content or key information and the recording/reproduction method must also be improved.